A compact disc (CD) is a high-volume and long lived data-storage medium. One recordable compact disc (CD-R) contains a polycarbonate disc that is coated with a dye layer, a metallized reflective layer, and a protective layer. A CD-R will be understood to be a compact disc that can be written on, typically by a laser beam as contrasted with a CD-ROM which information is recorded by injection molding. Cyanine, phthalocyanine, and metallized azo dyes are commonly used dyes coated in a polymer binder in the dye layer. The metallized reflective layer typically consists of gold in CD-R, and aluminum in CD-ROM. In a CD writer, a laser beam illuminates the dye polymers through the polycarbonate substrate as the disc spins. The illumination is turned on and off at selective locations determined by the input digital information. The heating by the laser causes the dye layer to chemically change at these locations, forming readable marks in the dye polymer. The degraded dye polymers in the marked regions are less reflective than the unmarked regions. During the reading process, a low-power laser scans the dye polymer layer in a recorded disc. The laser light is reflected directly from the unmarked regions, but is scattered or diminished in the marked regions. A sensor monitors the transitions between the marked and unmarked regions from the intensity of the reflective light, and converts it into a digital data stream. Similar to the above process, a CD-ROM differentiates the intensity of the reflective light by pits and lands in the compact discs. These pits and lands are pre-recorded by pressing the compact discs, typically mass produced.
The CDs are often coated with a printable surface opposite to the surface from which the information is recorded and retrieved. On the printable surface, a label is printed which can be logos, trademarks, text, graphics, and bar codes, etc., which are related to the information stored on the CD. The label also protects the CD from physical damage. Because the CD spins at high speed in the writer and the player, the CD label needs to be precisely balanced to the center of the disc for smooth rotation.
Labeling of CD discs has routinely been accomplished through screen printing methods. While this method can provide a wide variety of label content, it tends to be cost ineffective for run lengths less than 300-400 discs because the fixed cost on unique materials and set-up are shared by all the discs in each run. The screen printing technique is well described in the textbook "Graphic Arts Manual", edited by Janet and Irving Field, Arno/Musarts Press, New York, N.Y., 1980, pp. 416 to 418. In screen printing a stencil of the image is prepared, placed in contact with the CD and then ink is spread by squeegee across the stencil surface. Where there are openings in the stencil the ink passes through to the surface of the CD, thus producing the image. Preparation of the stencil is an elaborate, time consuming and expensive process.
Recently, significant increases in use of CD-R discs as a data distribution vehicle have increased the need to provide customized CD label content to reflect the data content of the disc. For these applications, the screen label printing presents a dilemma as CD-R discs are designed to allow customized user information to be recorded in standardized CD formats.
Initially, the customized label information was "hand written" on the disc surface using felt tipped markers. While this method allowed users to individually identify discs, it tends to be labor intensive, prone to human error in transcription, and aesthetically limited.
Other attempts to provide a CD-R labeling solution has incorporated digitally printed adhesive labels. Label stock for this type of CD-R labeling is available from a number of sources. These allow pre-cut labels to be printed using desktop or commercial ink-jet, thermal wax transfer, or electrophotographic printers. An example of such labels is the STOMP Company's (Irvine, Calif.) CD Stomper package of die-cut CD labels that can be printed on any 8.5 by 11 inch inkjet or laser printer. Following printing, the labels can be applied manually with or without the aid of an alignment tool or a specially designed machine. This method can be labor intensive. It is also prone to human error in label transfer. Damage to the CD-R can result if the label is removed. System performance problems can occur due to disc imbalance or label delamination in the CD writer or reader.
U.S. Pat. No. 5,317,337 describes an apparatus and method for printing label information on a CD. Both ink jet and laser printing are described, but the laser printing is limited to printing ink onto an intermediate drum and then transferring the image to the CD label, that is, offset printing
Within the past several years, methods for direct CD labeling have been growing in prominence. These methods utilize the versatility and ease of the setup associated with digital printing to provide customized label content directly on a disc surface. The most commonly used direct CD printers incorporate ink jet or thermal wax transfer technologies. Examples of such printers are the AFFEX Corporation's (2522 Chambers Road, Suite 110, Tustin, Calif.) Multi Media Color Ink Jet Printer, the FARGO Corporation's (Eden Prairie, Minn.) Signature CD Color Printer. These printers can be either stand alone or integrated into a computerized disc writing system reducing problems associated with labor, human error, disc damage, and imbalance. While printers of this type can produce satisfactory output, specially designed layers are required for their use. There is concern over performance of printed image quality for both types of printers. Thermal printing has demonstrated a lack of robustness with respect to abrasion and ink jet printing is less resistant to moisture. There are additional concerns over the inability to produce multi-color output on the thermal wax transfer CD-label printers, and the long print time required for the ink jet label printing. Additionally, both of these printers are binary in the density scale, and cannot reproduce continuous tone photographic images.
One known continuous-tone digital color printing technique is the thermal resistive dye diffusion (or sublimation) printer. Printing techniques have been disclosed in U.S. Pat. No. 5,542,768, and the above cross referenced co-pending applications. However, a thermal resistive head (both thermal wax transfer and dye diffusion) prints at a pressure contact to the CD surface. Good printing uniformity by thermal resistive printing requires (see for example, U.S. Pat. No. 5,244,861) a conformable layer in the receiving paper, which is lacking in CD-R discs.
Screen printing is not economic for printing label images on a small number of discs. Ink-jet and thermal wax transfer printing methods are binary, and therefore not suitable for photographic quality continuous tone printing. Thermal resistive head printing techniques including wax transfer and thermal dye diffusion printing require either a conformable surface or a high pressure nip contact between the print head and the Photo CD surface, both of which make the process expensive and difficult. All the above techniques, to different degrees, are slow in printing speeds.